1. Field of Invention
This disclosure describes the means to transmit two signals on the same frequency at the same time while some of the receivers may be mobile and without tracking and others fixed. The invention is intended for use in radio frequency applications, and is intended to be used where information is being transmitted on radio frequency signals.
2. Prior Art
Frequency reuse means the simultaneous transmission of more than one signal on any particular frequency. This is usually accomplished by sending the signals in two different polarities. Examples are: horizontal and vertical polarization; and left and right circular polarization. The two signals do not interfere with each other because the polarities do not interact, allowing two signals to be transmitted in the same frequency. The receive antenna needs to have the same polarization scheme (linear of circular) and needs to be aligned with the transmitting antenna in order to be able to pick up each of the two transmitted signals separately.
One of the problems with communications between a mobile station and a fixed (base) station is that the polarity of the receiving station rotates in three axes in relation to the fixed transmitting station. The receiving antenna has no way to discriminate between the two transmitting polarities, unless you steer the receiving antenna and associated feed to align the antenna's polarity with the transmitting antenna. The receive antenna's polarity needs to be precisely aligned with the transmit antenna's polarity, precluding frequency reuse in mobile systems that do not have tracking. Tracking systems are not compact enough or cheap enough to incorporate into low cost mobile communication systems.
Consequently, when a base station transmits to a mobile receiving station which does not steer its antenna, only one signal instead of two can be transmitted. If two polarized signals on the same frequency were transmitted, both signals would be picked up by the receiving antenna. The power level of each signal would depend on the relationship of the polarization of the transmitting antenna to the receive antenna. Decoding either of the two signals would be difficult or impossible.
The technology described below allows for the use of one polarization for mobile (and fixed) uses and the other polarization for fixed uses, thus allowing for reuse of the same frequencies while maintaining the ability to transmit to mobile users. Bandwidth is tightly controlled by the FCC and in short supply, necessitating frequency reuse wherever possible.
The advantage of this means of communication is the efficient use of bandwidth. Most other techniques for improving bandwidth utilization (CDMA, spread spectrum, TDMA, demand assigned access, etc.) can be used in combination with this means.